Photoengraver&#39;s powdering cabinet



Dec. 31, 1957 o. T; SMITH ET AL 2,- 8 'l8,( )43 -PHOTOENGRAVERSPOWDERING CABINET Filed March 18,. I954 3 Sheets-Sheet l? INVENTORS OTTOTELFA/R SMITH BY GORGE A. BUTTER, JR- Wa ATTORNEYS- Dgc. 31,1957

o. T. SMITH ET AL 'PHOTOENGRAVERS POWDERING CABINET 3 Sheets-Sheet 5Filed March 18, 195

R H wn 5 8 5 WM N e N r R mew w Nm T I 0 MA @WA W W TM 0 H Y M UnitedStates Patent 'PHOTOENGRAVERS POWDERING CABINET @tto Telfair Smith,Muttontown, and George A. Butter, Jr., Levittown, N. Y., assignors toPowers Chemco, Inc, Glen Cove, N. Y., a corporation of New YorkApplication March 18, 1954, Serial No. 417,090

9 Claims. (Cl. 118-11) The present invention relates to powderingcabinets used by photoengravers in powdering partially etched platesbefore further etching and has specific reference to such a cabinethaving means to prevent powder escaping from the cabinet to atmosphereduring the powdering operation.

Objects and advantages of the invention will be set forth in parthereinafter and in part will be obvious herefrom, or may be learned bypractice with the invention, the same being realized andattained bymeans of the instrumentalities and combinations pointed out in theappended claims.

The invention consists in the novel parts, constructions, arrangements,combinations and improvements herein shown and described.

The accompanying drawings, referred to herein and constituting a parthereof, illustrate one embodiment of the invention, and together withthe description, serve to explain the principles of the invention.

In the drawings:

Figure 1 is a perspective view of the powder cabinet embodying thepresent invention and being partially broken away to show the mechanismwithin the interior of the cabinet;

Figure 2 is a vertical sectional view of the cabinet takengenerallyalong line 22 of Figure 3;

33 of Figure 2; and

Figure 4 is a diagrammatic view of the electrical circuit which forms aspart of the present invention.

In preparing partially etched plates for further etching a dark redresin, known as dragons blood, is dusted onto the plates. In dusting theplate, a flat brush charged with the powder is swept across the plate infour different directions varying approximately 90 with the plate beingheated after it has been dusted in each direction, this heating meltingthe powder so it becomes fused and forms an acid resistcnt coating. Bydusting in this manner, the powder is effectively appliedto thepartially etched image so that when the plate is treated with acid forfurther etching the relief will be increased without undercutting orlosing the detail.

During the dusting or powdering operation the fine powder is agitated sothat particles of the powder rise into the air in the fashion of dust.For the safety of personnel as well as other reasons it is essentialthat this powder does not escape and become lost to the atmosphere. Itis for this reason that the powdering operation is conducted inspecially designed cabinets.

The present invention has for its objects the provision of an improvedcabinet for use in powdering partially etched plates and wherein thepowder that rises from the plate during the powdering operation isrecovered by means of a downdraft air flow system without disturbing thequiescent atmosphere in the vicinity of the plate or in any wayinterfering with the powdering of the plate. Another object is animproved powder cabinet having a recovery system for the powder risingfrom the plate In accordance with the illustrative and preferredembodiment of the invention the powder cabinet comprises a base on theupper end of which is a box-like hood provided with spaced vertical sidewalls interconnected by spaced vertical front and rear walls. The top ofthe hood is entirely open and the front wall is considerably lower thanthe rear wall with the upper edges of the side walls preferably slopinggradually downward from the Posi-.

top of the rear wall to the top of the front wall. tioned within thehood is a powder tray of generally the same configuration as the hoodbut smaller in all respects so that it is contained entirely within thehood with the upper edges of the walls of the tray being below thecorresponding edges of the hood and with the walls and bottom of thetray being spaced from the walls and bottom of the hood therebyproviding a narrow passage for air flow all the way around the tray. Theinterior of the hood and the interior of the base are separated by aplate which forms the bottom of the hood and which is provided with apair of openings each of which has an upright fabric filter bagpositioned therewithin and depending into the base of the cabinet.Within the base is a fan driven by an electric motor and adapted toapply asuction. to the interior of the base causing aflow of airdownward through the passage surrounding the powder tray, into the openupper end of the filter bag, and through the wall of the bag into theinterior of the base from which it is exhausted by the fan toatmosphere. I

In flowing through the wall of the filter bag the powder, that isentrained in the air is removed and remains in the interior of the bag.Attached to these filter bags is an electric vibrator operative whenenergized to shake the bags to remove the powder that clings to theinner wall of the bags, this powder falling into the bottom of the bagleaving the walls relatively clean for eflicient filtering operation.and the vibrator is controlled through suitable electric circuits whichinclude a switch means operable when in one position to energize themotor and de-energize the vibrator and when in another position totie-energize the motor and energize the vibrator. In the circuitassociated with the vibrator is a switch automatically operative tode-energize the vibrator a predetermined time after it has beenenergized.

Referring now to the drawings, wherein like reference characters areused throughout to designate like elements, the illustrative andpreferred embodiment of the powder cabinet of the present inventionshown therein comprises base It) provided with side walls 14, rear wall16 and bottom member 18. The inner portion of the front 20 of this baseis recessed as shown and provided at its upper end with a receptacle fordrawer 21 beneath which is access opening 22 normally closed in a fluidtight manner by removable plate 24. The front wall 20 and side walls 14of base 10 extend below bottom member 18 and provide a footing for thebase in engagement with the floor upon which the cabinet is placed withpassage 23 being formed between the bottom member and the floor andleading out the back of the cabinet.

Upon the upper end of base It) is formed the hood portion of thecabinet, this hood portion being generally designated 12 and includingbottom plate 25 preferably positioned on top of base 10 and forming whatmay be termed the roof of the base with the front end of the plate beingbent upwardly to form the front wall 26 of I the hood. The side and rearwalls of the hood are Energization of the electric motor formed byupwardextensions of side walls 14 and rear wall 16- of base 10 with the rearwall 16 ofthe hood being substantially higher than front wall 26 asshown and with the upper edges of side walls 14 sloping gs 'aduallydownward from thetop of the rear wall to. the topofi the-front wallthereby giving the operator a maxi.- mum of working freedom whiclrpromotes-both production and quality. 7

Mounted within hood 12 is; powder tray 27 havingside walls 28, rear andfront walls 30 and 32, respectively, and bottom 34. The tray ispivotally mounted about rod 36 which extends through a suitable beadformed atthe'upperedge of rearwall30. and has its opposite; ends securedto-side-wallsfl of the-hood"; When the powdertray is i'n-its normal oroperative position, shown in ,full lines i'rrFi'gure 1 and'dottcd linesin Figure 2; the head or loop provided on the upper edgeoffrontwall; 32is in engagemeutwith stop member 38 secured to side wall 14 of the hoodand bottom 34 is inparallel; relation with but spaced from bottomplate25 of the hood forming passage 4-0 tlierebetween. The side walls 28' andfront and rear walls 32' and 30 of thetray' are respectively spaced'from sidewalls 14 and front: and rear walls 26 and 16 oh the hoodthereby formingannular passage 42- about" tha tray and communicating: atitslower end withpas sage: 40:;

For the purpose of' somewhat guiding tray 27 inits pivotal movementabout rod 36 and for-retainingthe tray in: its extreme upwardly tiltedposition bracket; 44 has one-end pivotally mounted to-the bottomofthe-tray with its other end pivoted to member 46 whichis in' turn.slidably mounted on track 48; As tray27 is'tilted upwardfrmemben 46slid'es along tracle48 until bracket 44 is substantially verticalwhereupouthe-bracket will hold the tray in this; position. the; powderin the tray collects. at-the back of thetray makingt'it relatively easy:to remove the powder with this removal being facilitated by plate 50which extends acrossthe; lower: rear edge -of the tray.

Rivotally mounted on the upper edge'ofi-f'rontwall 26 of the hood isshelf 60 which is movable between its full line position in Figure. 2'where it permits pivotal movement' of tray 27' and its fulllinepositionin Figure 1 where it extends over and is spacedfrom the upperedge of thefront-wall of the tray andprevents foreign matter falling in; the:spacevbetween the front wallsof} the'tray and the hood.

Inlbottom plate 24' is: a pairof openings 52 within-each of; which, isan upright close mesh fabric filter bag 54 preferably made of nylonandWhich extends downwardly intoithevinterior of base 10; Therimf'bag'54'is' formed" with, a rigid. annular member. 56. disposedtherein with this; rim being slightly larger than opening 52therebyforming a radial flange etfective=to support the bag. To prevent:the bagsefrom collapsing and becoming out of shapeseach oithernhas-1mhoopfifi positioned in thebot-- tom.

During: the'powdcring of a.plate=.within tray 27, suchasaplatettil'heldby'hand or: placedeontshelf- 60; a continuous downdraft is createdin. annular passage 42 by fan- 64 mounted within base 102 andrhaving itsinlet communicating; with the interior of:the base: and its outletzcommunicating with passage 23. The base is sufii-- By: tilting the trayin this-manner By creating a downwdraft through annular passage 42 theentire tray is surrounded by a current of air moving toward the passage42 with the air over tray 27 moving generally outwardly from the centertoward each of the walls whereby no rush of air above or across the trayfrom one end to the other is created as has been the case heretofore andwhereby. the quiescent atmosphere necessary for quality work isundisturbed while dust collection efficiency is greatly improved.

In: order, to maintain the. ethciencyof filter bags 54: at their maximuma shaker-is provided to periodically shake the bags to'removetthepowderthat clings to-the interior wall ofther bags with this powderfalling to-and collecting in the bottom of the bag. The bags areofsufiicient size so that under normal operation they need emptying onlyabout twice a year with the emptying operation being accomplished bytilting tray 27 to its uppermost position and then removing the bagsupwardly out of the openings 52.

As illustratively disclosed the means for shaking bags 54 compriseselectromagnet 66 having its armature 67 normally biased to anonvenergized position by spring 68 and connected with the bags by crossbracket 70 pinned to the radially extending ears 72 which form partofannulan frame 74' secured to each of the bags. EX- tending laterallyfrom armature 67 is bracket 76 which has a pair of fingers 78' betweenwhich is positioned switch 80'forminga' part of the circuit ofelectromagnet 66 and'arranged to open and break the circuitvwhenarmature 67 is moved to its extreme position within the coil of theelectromagnet in responsehto .energization of the electromagnet and toclose when' armature 67is moved to its other extreme position by spring68 with the limit of thesetwo extreme'positionsbeing determined by stops82 which engage bracket 76'. Thus when the circuit of el'ectromagnet 66-is energized armature 67 will rapidly move back and forth'between these'extreme positions because of the opening and closing of switch 80thereby resulting in shaking filter bags 54;

The operation of' fan 64- and electromagnet 66 is controlled by anelectriccircuitdiagrammaticallyillustrated in Figure 4 and arranged sothat when the-motor-84 of the fan is energized electromagnet 66"will notbe energized and can only be energized after motor 84 has first beenenergized'and then de-energized. Electromagnet 66 will be energized onlyfor a predetermined time after which it will automatically bedeenergized. This circuit comprises a pair ofl'eads86and 88 connected witha suitable source of" potential and across whichare connected motor84'and-coil of relay 90inparallel relation with; each other with thecircuitof-motor 84 including conductors'92 and 94 andthe circuit ofrelay 9fl=including conductors 96' and 98 Forminga portion of theconductors 92 and 96i's switch 100=whichis preferably a double pole,double throwswitch of' the toggle type without an ofiposition. Whenswitch: 100* is moved' to the right, as viewed in Figure 4, intoengagement with-com tacts 102 and 104, conductors 92'and 96;respectively, are completed while when it-' is moved to-its left inengagement with contact 106 both of these conductorsare brokenandtcontact 106 is connectedto relay-90 through a portion of conductor96.

Thus when switch 100 is moved 'to its right position motor 84 and relay90 arelenergized. Energization of relay 90 causes normally open'contacts108 and=110 of the relay to close and establishes through conductor 112a holding circuit for the relay in parallel with-switch 100 so that whenthe switch is'moved to its left position relay 90 will remainenergized-:wliilemotor will become de-energized.

The circuit for electromagnet 6'6' includescontact'106; conductor 114; aseries connected switch 80* and conductor 116 which is connected tolead-88. Whemswiteh 100 is'moved fromits right to itsleft-positioninengagement with contact 106' this circuit' of)electromagnet 66' asiaoas is completed across leads 86 and 88 sincecontact 106 is then connected with lead 86 through conductors 96 and 112and contacts 108 and 110. However, should relay 90 become de-energizedcausing contacts 108 and 110 to open the circuit of electromagnet 66would of course become de-energized.

In series with coil of relay 90'and forming a part of conductor 98 isthermal switch 118 which comprises bimetal element 120 in engagementwith contact 122 at normal temperature and movable out of engagementwith the contact when its temperature is raised to a predeterminedvalue. Immediately adjacent to bimetal element 120 is resistance heater124 connected across conductors 114 and 98. No current can flow inresistance 124 until contacts 108 and 110 of self-locking relay 90 areclosed which is accomplished by throwing the switch 100 to the right.Resistance 124 then is in a position to receive current when switch 100is thrown to the left hand position making contact 106 which completesthe path to resistance 124 namely conductor 88, thermal switch arm 120,contact 122, conductor 98 to heater,

conductor 114, contact 106, switch 100, conductor 96,

conductor 112 and contacts 108 and 110 to conductor 86. When switch 100is thrown left connecting to contact 106 the circuit of electromagnet 66is energized, current will flow from conductor 114 through theresistance heater to conductor 98 causing the resistance heater tobecome hot and thereby heat bimetal element 120 resulting in switch 118being opened which will in turn de-energize electromagnet 90, openingcontacts 108 and 110, with resulting de-energization of the circuit ofelectromagnet 66.

Since switch 100 is preferably of the toggle type it will at all timesoccupy either its right position in engagement with contacts 102 and 104or its left position in engagement with contact 106.

In operating the cabinet of this invention, when it is desired to powdera partially etched plate switch 100 is first moved to the rightcompleting the circuit of motor 84 thereby causing fan 64 to draw airdownward through passage 42 and also completing the circuit of relay 90.Energizing of relay 90 causes contacts 108 and 110 to close therebycompleting the holding circuit of the relay which is in parallel withswitch 100. After the powdering operation is completed and it is nolonger necessary to run fan 64, switch 100 is moved to the left intoengagement with contact 106. This movement of switch 100 de-energizesmotor 84 but because of the holding circuit does not de-energize relay90. Upon moving the switch into engagement with contact 106 the circuitof electromagnet 66 is completed resulting in shaking of the filter bags54. Also incident to energization of the circuit of electro-magnet 66resistance heater 124 begins to get hot and after a predetermined time,depending upon the resistance value of the heater, it will have heatedbimetal element 120 sufiiciently to cause it to move away from contact122 which will de-energize relay 90 and consequently causede-energization of the circuit of electromagnet 66. Thus filter bags 54will be shaken only for a short time sufiicient to remove the powderfrom the inner walls thereof after which the electric vibrator will beautomatically de-energized.

The invention in its broader aspects is not limited to the specificmechanisms shown and described but departures may be made therefromwithin the scope of the accompanying claims without departing from theprinciples of the invention and without sacrificing its chiefadvantages.

What is claimed is:

l. A powder cabinet of the type described comprising a hood forming theupper portion, said hood having an open top and a front the majorportion of which is open, a powder tray having side, rear and frontwalls and providing a normally horizontal working surface disposedwithin said hood, said tray being smaller than said hood 6' so thatpassages are formed between the side', rear and front walls of the trayand the hood and the tray is positioned entirely within the hood, meansin the lower portion of said cabinet effective to draw air downwardlythrough said passages, and separator means through which said air isdirected for separating foreign matter therefrom.

2. A powder cabinet for use in preparing plates for etching comprising agenerally upright base, a hood mounted on said base and forming theupper portion of the cabinet, said hood being open on the top and havingelements comprising a bottom and generally upright walls enclosing thefront, rear and sides thereof with the front wall being substantiallylower than the rear wall and with the edges of the side walls slopingfrom the rear wall downward to the front wall, a powder tray providing anormally horizontal working surface and being of generally the sameconfiguration as said hood and disposed entirely within said hood, thebottom and walls of said tray being spaced from the correspondingelements of said hood thereby forming a narrow passage for airflowentirely surrounding said tray, said tray being pivotally mountedadjacent the rear wall of said hood about an axis extending along theupper edge of the rear wall of said tray the bottom of said hood havingan opening therein, a fan disposed in the base of the cabinet andeffective to apply a suction to said passage through said openingcausinga downward flow of air through said passage on all sides of the tray,and an upright close mesh fabric filter bag removably disposed withinsaid opening for removing foreign matter from the air drawn through saidopening.

3. A powder cabinet as defined in claim 2 including a shelf hingedlymounted on the upper edge of the front wall of the hood and positionableso as to extend toward the rear of said hood beyond and above the frontwall of the powder tray.

4. A powder cabinet of the type described comprising a base, a hoodpositioned on the upper end of said base, said hood having a generallyhorizontal bottom member, a pair of spaced side walls extending upwardlyfrom said bottom member, a front and a rear wall extending upwardly fromsaid bottom member intermediate between and interconnecting said sidewalls, said front wall being substantially lower than said rear wall andthe upper edges or" said side walls sloping gradually downward from thetop of the rear wall to the top of the front wall, a powder tray havinga bottom and side, rear and front walls disposed within said hood, thebottom of said tray providing a Working surface and being spaced abovethe bottom member of the hood and the walls of the tray being spacedfrom the walls of the hood thereby forming a narrow passage completelyaround said tray, the upper edges of the Walls of the tray beingpositioned below the corresponding edges of the walls of the hood, saidtray being pivotally mounted with respect to the rear wall of said hoodand about an axis at the upper edge of the rear wall of said tray thebottom member of the hood having an opening therein, an upright filterbag disposed Within said opening, substantially filling the same anddepending into the base of the cabinet, and a fan in the base of saidcabinet effective to draw air downwardly through the passage around saidtray and into said bag and through the wall thereof.

5. A powder cabinet comprising an upright base, a hood having side, rearand front walls and mounted on said base and forming the upper portionof said cabinet, said hood being adapted to have a powder tray providinga working surface disposed therein with the peripheral extremities ofsaid tray spaced from said hood walls to provide a passage for air flowsurrounding said tray, a fan in the base of said cabinet effective todraw powder laden air throughv said hood into said base, an upright air"enters" the =-open upper end of the bag and passes throughthe-wall ofthe bag, an electricrnotor -connected to said fan, switch*rneahwmovablebetween a first and-asecondposition and being' eonstructedand arranged to occupy one of said positions at all times, said switchmeans having a pair of'contactv forming apart 'of the circuit of-saidmotr said contacts .being 'closed'whentlre switch means is-in oneo'fisaid positions and =-openwhen in the =otherof said an -electric-vibrator "connected to said bag and tipentive'to'vibra tethe same whenenergized; said switch means having; again of contactsforminga partofithecirctfitofisaid vibrator; thelast named contacts being closed whenthe-switch means is in i said otherposition-"and open when in said oneposition, and means operative a predetermined time'after energization ofsaid vibrator to de-energize the same.

6. Apowdercabinct comprising-a base; 'ahood mounted on said baseand--havingfront;rear and sidewalls, 'a powder tray providing aworkin'g'surf'ac'e disposed within-said hood andwhaving its periphera'lextrernities-spaced from said' hood walls'to form --an*air 'flowpassage'snrrounding said tray, an -upright fitter-bag in-saidcabinetpafan operative to -draw powder *laden air through saidhood airflowpassageyinto the open-upper end -of the filter bag and throughthewal-l some bagflmeans fondriiring said fan including -an-electricmotordisposed i-n'an electric circuit adapted to beconnected'across-apotential; an electric switch means in said'circuit and movable betweena first and a second position hocauseenergization and de-energizationrespectively of said circuit, said switch means being constructedand'arrangedtb-atall "times o'ccupy one of said positions-asecond-icircuit inparallel with the first mentioned circuit andincludingan electric vibrator 'operatively-con'nected to said filter bag, saidswitch means including means efiective to makeand break saidsecond-circuit whenasaid sw itch means occupies its second andfirst-positions respectively,additional means in saidsecond circuiteffective to control energization thereof, said means being conditionedin response to said switch means occupyin'g' its-said first position topermit energization of saidsccond circuit, means operable to maintainsaid controhm'eans so conditionedpwh'en said switch 'means is movedfrom-its first to'its second position, and .means effectively responsiveto'energ'ization of said second circuit and operableafter said secondcircuit has been energized for -a predeterrnineddime-to condition saidcontrol means it to prevent 'energization ofsaid second circuit.

7. Apowder cabinet comprising a base, a hood mounted on said base andhaving-front, rear 'andside walls, a powder tray providing a workingsurface disposed within said I hood and having its peripheralextremities spaced from said hood walls to form anairflow passagesurrounding said;tray, a close mes'h'filter bag in said cabinet, a fanoperative to draw powder laden air throughsaid passage, into the openend of thefilter b'agand through the wall of the bag, an electriccircuit including-amotor'connected to drive said fan,switch-means havinga pairof contacts in said circuit, 'said' switch means being movablebetween a'first and second position to respectively close and opensaidcontacts, a second circuit in parallel with said motor and includinga pair of contacts forming a part of said switch means, said 'contactsbeing in engagement when the switch :meansislin-saidfirst-position:andseparated when in said second position, a relay having its coil inseries with, said second icircuit, the contacts of said relay beingnormallyiopen and 'being connected to form a holding :Icircuit :forithe'zrel'aytin iparalle'l with "said contacts :in ;said tsecond'circnit, a :normally closed therrnal switch .iniseriesiwith,=said-:second circuit and said holding circuit, said swi ch:tmoying ito tan openiposition upon reaching a predetermined hightemperature, a mag netic vibratoroperatively connected to saidfilterbag, said vib rator'being in-series with said holding circuit andparallel with said relay coil andsaid thermal switch, saidswitchmeansincludihgmeans'operative toopcn the vibrator circuit whensaid switch means is in said 'firstposition' and-: completesaid vibratorcircuit-when said switch means-is'insaid second position, an electricresistance heaterjuxtaposed to' said thermal switch and connected inseries'withthe last mentioned means and said thermal switch.

8. A powder cabinet 'used in preparing plates for etching "comprising abase, a hood'disposed .on the upper end ofjsaid base, said hood havinganopen top, and generally vertical 'walls'with the front wall beingsubstantially lower than the rear wall, the interior of said hood'being,separatedffrom the interior of said base by a plate member having anopening jt'herethrough, a powder tray providing.

a working surface disposed within said .ho0d,-.said,'.tray

having a bottom and generally vertical walls spaced, from,

and dependingjnto .said, base, .an electric circuitinclud ing amotortfordriving said 'tfan, antelectric .circuitincluding avibratortconnectedsto. said :bag, and effective to vibrate the samewvhenenergized-switch tmeans tdisposcd in.thetcircnit. -.ofiisaid motorandythe circuit of said vibrator and movable between a first andzasecond position,

whereinwit is effective to respectively, makeand break the circuit-ofthesmotortand break: and make the circuit oftthe vibrator, and .meansresponsive tto energization of,

said vibrator .cincuit'c-and operable to breaktsaid vibratorcircuittafter energization1ofthe-same-for.a predetermined,

time.

9. .A powder-cabinet thaving a hood formingrthe "upper portion thereof,said hood'ha-ving sideand rear walls, ,an open topand altront ,themajorportionaof which;is open,

a powdcntray havingside, =rear and rfront wallsiand substantiallycentrally disposed within said .hood, :said 'tray providingazworkingsurface :andbeing generally iOf the same configuration !as said hood butsmallertin allrespects so as totorm an annular passage aboutsaidtrayvbetween,

the walls oftthetray; and ithe hoodtand so the:tray ispositioned:entirelyrwithin -thethooi-a -fan in ,the lower position ofthe .cabinetoperativeato ,applya suction to :the lower end of,,said-.passa ge zandtdrawipowder laden air downwardly theret'hrongh, ,anupright fabricfilter bag disposed in .the path of 1 said powder ladeni-air; means ,for driving said fan including;antelectrictcircuitiadapted to be iconnectedacross apotential, an: electric; switch :meansin said circuitwandmovable. between a :first :and, a second positiontozcauseenergization and-de-energizationrespectively of said.circuit,said.-switch means being constructed and arranged to at alltimes occupy one,of;said positions, a second tcircuitdn parallelwithtthe :first mentioned tcircuit and including zan'zelectric -vibrator,operatively connected to said filter bag,.said switch means includingmeans-effectiveto (make andibreakssaid second circuit when said switchmeans :occupies iits second and first positions 'respectively,additional means in said second circuit effective to controlenergization thereof, said means being 'conditionediin-iresponse' tosaid switch means occupying its saidzfirst position to permitenergization 'of -said second circuit, ,rneans operalile tomaintain'saidcontrol means so energization 'ortsaia second circuit andoperable after tion of said second circuit.

References Cited in the file of this patent UNITED STATES PATENTSDobinson Dec. 31, 1929 10 Merrell et a1 May 27, 1930 Guenst Sept. 12,1933 Black May 30, 1944 Smith June 19, 1951 Medcalf July 8, 1952Bruckner Mar. 31, 1953

